Lung cancer is the leading cause of cancer mortality in the United States, with an incidence of approximately 213,000 new cases per year and a very high mortalityl15. Despite new drugs and therapeutic regimens, the prognosis for lung cancer patients has not changed significantly in the last 20 years, emphasizing the need for novel treatment strategies. Targeting the epigenome, represents a promising therapeutic strategy in cancer16.
Aberrant DNA methylation has been shown to play important roles in lung cancer17:
1) promoter methylation is one of the mechanisms responsible for silencing TSGs18-20, such as CDKN2A, CDH13, FHIT, WWOX, CDH1, and RASSF1A;
2) mRNA expression of the maintenance and de novo DNA methyltransferases, DNMT1 and DNMT3B, respectively, were reportedly elevated in 53% and 58% of 102 NSCLCs, respectively, and the DNMT1 mRNA level was shown to be an independent prognostic factor for survival13;
3) DNMT1, DNMT3A and DNMT3B protein expression is elevated in lung tumors relative to normal lung tissue12;
4) a specific polymorphism in the human DNMT3B promoter, which significantly increases the promoter activity, has been associated with increased lung cancer risk21;
5) inhibition of DNMT1-mediated DNA methylation reduced tobacco carcinogen-induced lung cancer in mice by >50%22.
MicroRNAs (miRNAs), non-coding RNAs of 19-25 nucleotides that regulate gene expression by inducing translational inhibition or cleavage of their target mRNAs through base pairing to partially or fully complementary sites, are involved in critical biological processes, including development, cell differentiation, apoptosis and proliferation1,2. Recently, specific miRNA expression profiles, with diagnostic and prognostic implications, have been identified for specific cancers (refs. 3-5 for review). Notably, members of the miR-29 family, previously shown to be down-regulated in NSCLC6,7, have been predicted in silico to be complementary to sites in the 3′ untranslated regions (3′UTRs) of DNMT3A and B genes, using different miRNA target gene prediction algorithms (PicTar8, TargetScan3.19, MiRanda10, and miRGen11) (FIG. 1).
Among the reported down-regulated miRNAs in lung cancer, the miR-29 family (29a, 29b, and 29c), has intriguing complementarities to the 3′ untranslated regions (UTRs) of DNMT3A and 3B (de novo methyltransferases)8-11, two key enzymes involved in DNA methylation, that are frequently up-regulated in lung cancer12 and associated with poor prognosis13.
While there is now believed that miRNAs play a role in carcinogenesis, miRNA expression is different in lung cancer versus its normal counterpart. Further, the significance of this aberrant expression is poorly understood.
Therefore, there is a need to determine whether miR-29s can target both DNMT3A and DNTM3B and whether the restoration of miR-29s can normalize aberrant patterns of methylation in lung cancers such as, for example, non-small cell lung cancer (NSCLC).